Electroplasticity in the Al_0.6CoCrFeNiMn high entropy alloy subjected to electrically-assisted uniaxial tension

Highlights

• Pulse current can decrease the flow stress and improve the elongation.

• Pulse current can enhance dislocation motion and inhibit dislocation multiplication.

• Local joule heating effect is the main electroplasticity mechanism in high entropy alloys.

• The lattice distortion effect results in the enhancement of the local joule heating.

Abstract

Electrically assisted deformation (EAD) was adopted in this work to overcome the shortcomings such as poor formability and easy cracking in the processing of dual-phase the Al_0.6CoCrFeNiMn high entropy alloy (HEA) at room temperature. Electroplasticity of the Al_0.6CoCrFeNiMn HEA was studied systematically using electrically assisted uniaxial tension. The results showed that pulse current caused the temperature gradient along the tensile direction and the temperatures of the samples increased with the current density. The flow stress decreased, and the elongation increased with increasing current density during the EAD. When the current density was 30 A mm^–2, the total elongation of the samples could be increased by 50% compared to that with no pulse. Pulse current can reduce local stress concentration and postpone microcracks initiation in the body-centered cubic (BCC) phases, and hence can effectively inhibit cracks and ruptures. The dislocation tangles were opened by pulse current, and the dislocation recovery was enhanced at a high current density. Compared with dilute solid solution alloys, the lattice distortion effect, the high fraction of the BCC phases, and the dislocations in HEAs can lead to the enhancement of the local Joule heating, which accelerated dislocation slip and dislocation annihilation. This study confirms that EAD can effectively improve the formability of HEAs and provides theoretical guidance and an experimental basis for forming HEAs components.

Introduction

Rapid developments in the aerospace industry have created a demand for alloys with superior properties. One option that provides a novel design method for materials [1, 2] is high entropy alloys (HEAs), a new class of materials composed of multi-principal elements with excellent strength [3], [4], [5], superior fracture toughness [6], exceptional irradiation resistance [7], corrosion resistance [8], and wear resistance [9]. The face-centered cubic (FCC)-structured HEAs (e.g., CoCrFeMnNi HEA) exhibit excellent ductility, but their strength is generally not sufficient for engineering applications [10]. To improve the strength-ductility balance, Aluminum, a body-centered cubic (BCC) phase stabilizer with low density, has been added to inherently FCC-structured HEAs to enhance their strength by forming hard BCC phases [11, 12]. For Al_xCoCrFeNiMn HEA, a single FCC structure changes to an FCC + BCC structure in the range of 8 at.%–16 at.% of Al, and it transforms to a single BCC structure as the Al content is larger than 16 at.% [13]. Among them, the dual-phase HEAs have high strength but are difficult to form at room temperature [13], [14], [15]. Elevated temperature processes such as hot working or warm forming were adopted to improve the formability of materials [16]; however, long-term exposure to high temperature led to serious oxidation of components and adhesion between the mold and billet.

Electrically assisted deformation (EAD), which reduces flow stress by applying current to metals during deformation [17, 18], can significantly improve the formability of the components [19] and has been applied to the forming of aluminum alloys [17], [18], [19], [20], magnesium alloys [21, 22], and copper alloys [23, 24]. The elongation of aluminum alloy can be increased by as much as 400% by adjusting the electrical pulse parameters [25]. The rotation of a crystal lattice in the AZ31B magnesium alloy was affected by the pulse current during EAD [26]. In these cases, the grains rotate from the basal hard orientation to a softer orientation, thereby weakening the basal texture [27] and resulting in extreme dynamic softening of the stress–strain curves before necking. In addition, the pulse current can induce recrystallization and increase the plasticity of deformed copper alloys [24].

Up to now, the detailed mechanisms of electroplasticity are not yet clear due to the difficulty of decoupling the thermal and electrical effects. Rudolf et al. [28] attributed the mechanical properties of pure copper and iron under EAD to the Joule heating effect induced by free electron transport since there are no other factors reducing flow stress. Zhang et al. [29] confirmed the existence of electron force in the EAD process by observing the dislocation annihilation and regeneration in nickel-based superalloys using electrical in situ transmission electron microscopy and found that electron force can affect the mechanical properties of alloys by promoting dislocation motion, i.e., the electron wind theory. Liu et al. [30] concluded that electroplasticity results from local Joule heating and atomic bond weakening near the defects. The thermal and mechanical models of AZ31 magnesium alloy under EAD established by our team [31] have shown that Joule heating is the main factor affecting the deformation behavior under EAD, of which 10%, or even less, is contributed by the electrical effect. In addition, researchers have proposed other theories such as thermal expansion, skin effect, and magnetoelastic to explain electroplasticity behavior [32], while the Joule heating effect and the electron wind theory are still considered the main mechanisms [33]. It can be found from the previous studies that there are obvious differences in the deformation mechanisms of various alloys under pulse current.

Compared with conventional dilute solid solution alloys (DSSAs), HEAs contain more elements, which condition induces the high entropy effect, the lattice distortion effect, the sluggish diffusion, and the “cocktail” effect [34]. Previous studies have confirmed that these characteristics of HEAs will lead to Portevin-Le Chatelier (PLC) effect at lower temperatures under direct current (DC), resulting in a decrease in the elongation of HEA [35, 36]. All five solid solution atoms in the cantor alloy are activated and cooperative diffused, which is significantly different from traditional DSSAs [36]. The current studies are limited to the deformation behavior of single-phase FCC alloys under DC, and there is a lack of detailed studies on the mechanisms for the electroplasticity of HEAs under pulse current that may produce greater elongation than that under DC. Hence, whether the formability of HEAs can be improved by EAD needs further study, and it is of great significance to study the electroplasticity of HEAs for forming HEA components in the aerospace field.

In this work, the electroplasticity of the dual-phase Al_0.6CoCrFeNiMn alloy was investigated from the perspective of microstructural evolution and tensile behavior. The mechanical properties of the HEA were first obtained using uniaxial tension subjected to various current densities, and the influence of current density on the tensile behavior of this alloy was analyzed. The specimens with the greatest elongation (30 A mm^–2) and with obvious strain softening (60 A mm^–2) were selected to compare with the tensile specimen at room temperature (RT). Based on the difference in lattice structure between HEAs and DSSAs, the electroplasticity of the HEA as well as the effect of local Joule heating on tensile behavior during EAD were determined and discussed.